Temporal And Spatial Dynamics Of Carbon Use Efficiency And Its Driving Factors Of Mature Subalpine Dark Coniferous Forests In Western Sichuan

Carbon use efficiency(CUE)defined as the ratio of primary net productivity(NPP)to gross primary productivity(GPP),an important function parameter,reflects the carbon sequestration capacity of forest ecosystems.It can help to determine and predict the carbon sink/source of terrestrial ecosystems,which is a matter of widespread concern.Mature or old forests play an important role in maintaining biodiversity and long-term carbon equilibrium.Changes in mature or old forest CUE can largely reflect underlying mechanisms in forest carbon sink/source function change,and affect regional and global carbon cycle and ecological equilibrium.Subalpine dark coniferous forests in western Sichuan is the second largest forest area in China,with mature forests accounting for more than 60%.Studying on CUE for the mature forests not only helps to reveal formation mechanisms in forest carbon sink/source,but also provides theoretical and practical basis for ecological safety and near-natural forest management strategies in western Sichuan.With a focus on Abies fabri mature forests,we use biometric measurements,eddy correlation technique and the metabolic theory of ecology(MTE)to(1)study CUE seasonal dynamics in stand and ecosystem and their driving factors;(2)propose and verify the plant and forest CUE model and critical hypothesis;(3)get the CUE formation mechanisms of trees and forests along the elevation gradient in Gongga Mountain on a large time scale.The main objects are as follows to:(1)revealing CUE seasonal dynamics of tree layer and its different components in a fir mature forest and their driving factors.The CUE of tree layer,fine roots and stem in a mature forest have similar dynamics in different seasons,and they have a positive correlation with the average monthly air temperature or soil temperature.Precipitation has a negative effect on needle CUE,and CUE of each organ ranges from 0.30 to 0.60.The CUE of the tree layer in different seasons mainly depends on changes in that of the wood and fine roots,where fine root CUE has a negative effect on needle CUE.Tree stem and needle CUE all decreased significantly with tree diameter.There are significant differences in CUE for small,medium and large trees,which are concentrated around 0.54,0.43 and 0.34,respectively.(2)clarifying CUE seasonal dynamics of the ecosystem and its different hierarchies and its driving factors in fir mature forest.The carbon budget(GPP and autotrophic respiration)and CUE of a mature forest ecosystem in different seasons increase with monthly average temperature.The ecosystem CUE(CUEeco)in the whole year,growth season and non growth season is 0.32±0.02,0.34±0.06 and 0.21±0.16 respectively,and the stand CUE(CUEf)during these periods is 0.41±0.01,0.43±0.04 and 0.31±0.06 respectively.The CUE(CUEsh)of the shrub layer was maintained above 0.40 throughout the year,which has negative and positive correlation with temperature in growing season and non-growing season respectively.Soil still has enough space for carbon storage,and its monthly carbon sequestration capacity is measured by the ratio of NPP and heterotrophic respiration,which is positively correlated with soil temperature,and can be a slight carbon source in non-growing season.(3)analyzing the effect of CUE of different hierarchies on the CUE of ecosystem.At the annual scale,CUEtr and soil carbon sequestration capacity are positively affected by air and soil temperatures,respectively,and CUEsh increases with CUEtr,negatively affected by precipitation.CUEsh,CUEtr and NPP/Rh can explain 85%changes in CUEeco.In the growing season,CUEtr is the most important factor affecting NPP/Rh and CUEeco.NPP/Rh and CUEeco decrease and increase with CUEtr,respectively.CUEtr affected by temperature is the key to determine CUEeco,which can explain 74%change of it.In non-growing season,CUEeco is both determined by CUEsh and soil carbon sequestration capacity.Soil carbon sequestration capacity affected by temperature and precipitation is important for CUEeco and can explain 79%change of it.(4)Proposing and verifing the plant CUE model and the stand CUE critical hypothesis.Based on the allometric metabolism theory and the relationship between organism respiration and growth,we obtained an iteration growth model(IGM).On this basis,we further derived a plant CUE model,and proposed a critical hypothesis to explain changes in stand CUE.The CUE model reveals that plant CUE is determined by the ratio of plant's current biomass to their potential maximum biomass(m/M).Under the framework of MTE,plant CUE decreases with plant growth.In addition to growth,this ratio is also related to plant metabolic exponent b^*,maintenance respiration coefficient m_rand growth respiration coefficient g_r.The critical hypothesis hold that under the continuous effects of interference and competition,an increase in m/M is blocked,and its value has a greater probability of tending to 0.37.In the case of g=0.33,the forest stand or ecosystem CUE showed a normal distribution centered around a certain value of 0.40 and less than 0.40,respectively.The global mature forest stand and ecosystem CUE showed the normal distributions with mean values of 0.36±0.11 and 0.198±0.13,respectively,supporting the critical hypothesis(5)simulating the growth and CUE of trees at different altitudes in long time scale.At a vertical gradient of 2900 m to 3600 m,the average growth trajectory of Abies fabri trees in the past 100 years is in accordance with IGM(b^*=0.75),and the average CUE of trees at an altitude of 3300 m is consistent with the measured results.The average CUE dynamics of trees at all altitudes decrease with tree size,and the decline steepness of CUE is determined by tree potential maximum biomass M.On a larger time scale,M is related to the tree crown and sapwood affected by light competition,stem geometry and moisture.With the larger relative crown volume,or less sapwood component,tree CUE have the smaller decline steepness.Without considering the effects of other limiting factors,the maximum time for tree CUE to decline to 0 may exceed 2000 years.However,in practice,the shortage of nutrients caused by hydraulic restrictions and the limitation of tree lifespan may cause the premature reduction and direct termination of tree CUE.In addition,there are many different in individual tree growth trajectories,resulting in the fluctuation of CUE dynamics.(6)quantifying the effect of tree size structure on stand CUE in long time scale,and estimating the CUE of Abies fabri mature forest at different altitudes.Although the sensitivity of forest's potential maximum biomass X_maxin different seasons to temperature is the main reason for the fluctuation of stand CUE on temperature,on a larger time scale,forest CUE is more dependent on tree death and regeneration balance.The mature forest stand CUE tends to be stable due to the stable tree size structure over time.These results directly support the critical hypothesis and to some extent coordinate the contradiction between two views that forest CUE is constant and variable.By tree size structure information at various altitudes,on average,CUE for mature Abies fabri forests in Western Sichuan can be estimated to 0.42±0.062.Based on these results,this study show that the CUE of subalpine dark mature coniferous forests in western Sichuan should around at 0.40.Fine root turnover and stable tree size structure are the main reasons for these forests to maintain carbon sequestration and a higher CUE.At a smaller time scale,temperature promotes forest NPP and CUE,but at a larger time scale,tree size structure and functional traits are more important,which depends on disturbance and competition.At present,an increase in mature forest CUE has a positive impact on the soil carbon sequestration capacity and ecosystem CUE.Given that forest soil have a large carbon storage space,dark coniferous forest ecosystem should be carbon sink for a long time.